The use of contact lenses for the correction of vision difficulties is known. However, such devices have not been accepted by many people, in part because of the time often required by the eye to become accustomed to their use, the physiological intolerance that sometimes develops often after only short periods of use, or the inadequate visual acuity which may be provided by them. Consequently, a great amount of effort has been expended in attempts to solve these problems for the contact lens wearer.
These efforts have included modification in both the design of the lens and the composition of the lens material. Modifications in lens design so as to improve the fit of the lens to the eye has produced only marginal reduction of the physiological intolerance. Modification of the composition of the lens material has also only been partially successful.
Early contact lenses were made from polymethyl methacrylate (PMMA), a hard material that is not water-absorptive. PMMA lenses frequently cause a great deal of physical discomfort to the wearer. Lenses of PMMA, although they are readily machinable and have excellent optical clarity and durability, have a high level of physiological intolerance for many individuals partly because of the stiffness of the lens and partly because the eye becomes deprived of oxygen due to the low oxygen permeability of the PMMA.
Lenses of soft water-absorptive hydrogel materials, such as poly(2-hydroxyethylmethacrylate) or poly(vinylpyrrolidone), produce reduction of irritation but at the cost of reduced visual acuity and greatly increased maintenance of the lenses. When such hydrogel lenses contain a high water content they have a high oxygen permeability and are, therefore, comfortable to wear. The high water content hydrogel lenses, however, have poor durability and exhibit a tendency to become cloudy due to absorption of proteinaceous material.
Lenses of silicone rubber are easily molded, are said to possess a softness similar to that of the upper lid of the human eye and have a high permeability to oxygen. However, contact lenses of silicone rubber may cause a burning sensation in the eye because of the low thermal conductivity of silicone rubber. Also, such lenses have been reported to sometimes tighten over the cornea in a "suction cup" fashion that impedes movement of the lens. This permits entrapment of toxic products and debris under the lenses and prevents tears from washing away this debris and lubricating the eyes, thereby causing abrasion of the eye. Furthermore, silicone rubber, being lipophilic, mucophilic, and inherently non-wettable, attracts debris, e.g., proteins, lipids, mucoids, and the like.
It is an object of this invention to provide contact lenses which do not have the disadvantages of the polymethyl methacrylate, silicone rubber, and hydrogel contact lenses.